The present invention relates to an electrical power supply system provided with an overvoltage suppression function, and more particularly it relates to an electrical power supply system capable of suppressing an overvoltage arising when a circuit breaker is operated in said power system.
Overvoltages arising when a circuit breaker is operated include a closing switching surge which builds up an overvoltage when a charging voltage in a transmission line at the time of closing undergoes successive reflection as a traveling wave, and an interrupting surge which also builds up an overvoltage when a traveling wave due to a voltage drop in the transmission line at the time of a ground fault interruption undergoes successive reflection. Sometimes, the largest of these overvoltages reaches three times the phase voltage. Heretofore, against such overvoltages, the resistance closing circuit breaker and the resistance interrupting circuit breaker have been utilized in conjunction with the efforts for designing the insulation level of the transmission system sufficiently to withstand such overvoltages.
In order to increase the capacity of electrical power transmission systems in accordance with an increasing demand for a greater power supply, construction plans for a 1000 kV power transmission system have been in progress. In such ultra-high-voltage power transmission systems, it becomes more difficult economically to build a power transmission line which can satisfactorily withstand overvoltages generated when the circuit breaker thereof is operated. Thereby, it is contemplated to design an overvoltage suppression equipment to suppress the overvoltages generated when the circuit breaker is operated to an extent as low as possible so as to enable economical construction of power transmission lines. For example, in a 1000 kV system, it is demanded to hold the overvoltages down less than 1.6 times the phase voltage peak values. By way of example, against the overvoltages arising when a circuit breaker is closed in power transmission systems having a lower voltage than the above, a resistance closing circuit breaker which is closed via a resistance and absorbs the energy of a travelling wave has been employed. This system, however, is not effective for suppressing the overvoltages at the time of interruption. Thereby, a resistance interrupting circuit breaker capable of being interrupted via a resistance also at the time of interruption has been proposed. However, in a puffer-type gas circuit breaker which is most popular nowadays, a resistance current conduction time during interruption is substantially longer as much as about 30 ms. Further, it must withstand the out-of-phase closing and interruption duty which becomes twice as much as the duty voltage for the circuit breaker. Thereby, the resistance thereof becomes inevitably large-sized, and in addition, a complicated delay operation mechanism is required to actuate the resistance closing unit or resistance interrupting unit thereof. Furthermore, a formidable problem of system reliability is always involved therein that a failure of the resistance or the actuating mechanism thereof will result in a bus bar failure then paralyzing the operation of the substation.
Further, the inventors of the present application have found that the overvoltages also arise on the sound or nonfaulty transmission systems, the circuit breakers of which are not operating. However, in the prior art, e.g. Japanese Patent Laid-Open No.Hei-2-146930, Hei-2-106133 etc., the overvoltages arising in the sound transmission systems has not been considered.